This invention relates to an aluminum-magnesium alloy sheet product for use as container stock including food containers and beverage container ends. The invention further relates to chemical treatment of said sheet product with an alkaline-based cleaning solution.
It is known to chemically treat aluminum alloy sheet product with an etchant or cleaning solution before painting, electrocoating, stamping, or the like. Conventional cleaning solutions remove machine residues, grinding and polishing pastes, oils, lubricants and other contaminants from the product surface. A typical cleaning solution includes an alkaline (caustic or carbonate) base and one or more of the following: a chelating agent for holding dissolved aluminum in solution, an emulsifier, a surfactant and a foam control agent. Previously known cleaners also included one or more inorganic phosphates, such as the aluminum cleaning solution taught in U.S. Pat. No. 3,655,467. Said cleaners have since been declared environmentally unsafe in several states.
When chemically treating aluminum-magnesium alloy sheet product with an alkaline-based cleaner, an unattractive reaction product film forms on the surface. This film is usually whitish, consists essentially of magnesium oxide and reduces the brightness levels of the sheet product on which it forms. The amount of film that forms may be reduced by maintaining such treatment variables as cleaning solution temperature and concentration, etch rate and exposure time at their optimums, once experimentally ascertained. Mechanical maintenance of these optimums does not completely inhibit the formation of film, however. Such maintenance is also less predictable than desired and costly in terms of the monitoring equipment, testing equipment, sheet product and man-hours required to determine the aforementioned optimums for each alloy sheet product.
In Japanese Pat. No. 59/205,488, there is disclosed a method of removing magnesium oxide film from an aluminum-magnesium alloy product to improve the adhesion of coatings to said product. More particularly, the reference teaches treating said product, after degreasing and/or etching, with an alkali-based solution which includes a sequestering agent of either 1% ethylenediamine tetraacetic acid (EDTA) or 3% sodium hexametaphosphate.
In still other methods of chemically treating aluminum-magnesium alloy sheet product, magnesium oxide is allowed to freely form on the product surface during alkaline etching. It is then later removed by treatment with an acidic solution, such as phosphoric acid or nitric acid. Two-stage cleaning processes are costly, however, in terms of the additional solutions, treatment areas and monitoring required. It is generally known to use 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) in conjunction with aluminum and aluminum alloys. For example, U.S. Pat. No. 3,687,858 teaches adding between 0.00-0.2% by weight of HEDP to a solution which comprises: from 0.5 to 15% caustic alkali, from 0.005 to 0.2% of a polycarboxylic acid salt having more than 10 carboxyl groups; and from 0.003% to 0.1% barium, strontium or, most preferably, calcium. Treatment with the above solution supposedly has a favorable effect on the adhesion of paints to aluminum surfaces in many instances. The reference is not specifically directed to aluminum-magnesium alloy sheet product, however. In fact, U.S. Pat. No. 3,687,858 neither teaches nor suggests inhibiting the formation of magnesium oxide-containing film on or improving the brightness levels of the sheet product herein.
Hydroxyphosphonic acids, such as HEDP, are used to retard precipitation of aluminum corrosion products from phosphate-borate type, ethylene glycol-based antifreeze formulations, as in U.S. Pat. No. 4,320,023. HEDP also seals the surfaces of anodized aluminum alloys according to U.S. Pat. No. 3,900,370. Lastly, U.S. Pat. No. 4,485,027 discloses degreasing aluminum workpieces with a solution containing 60 g/l of 50% NaOH and 50 g/l of the following dispersion: 18% boric acid; 77.5% orthophosphoric acid; 0.5% alkylbenzenesulfonic acid; 2% of an ethylene oxide adduct on a nonyl phenol; and 2% of 1-hydroxyethane-1,1-diphosphonic acid (another name for "HEDP"). The latter reference acknowledges formation of a white coating on metal surfaces treated with an orthophosphoric acid-containing solution. It does not, however, teach or suggest inhibiting the formation of magnesium oxide-containing film on aluminum-magnesium alloy sheet product.
It is further known to include HEDP in a strongly alkaline cleaning agent for preventing the formation of calcium deposits from hard water. In Canadian Pat. No. 753,208, however, large amounts of HEDP, between 0.5 and 20% by weight, are required to clean stainless steel or glass bottles.
In U.S. Pat. No. 4,010,086, there is disclosed a method for electrocleaning metals, preferably steel. The method comprises positioning metallic articles in a bath and passing electrical current therethrough. More particularly, said bath includes a sufficient amount of an alkaline metal hydroxide and an effective amount of a cleaning agent selected from the group consisting of 1-hydroethylidene-1,1-diphosphonic acid (HEDP), an alkali metal salt of HEDP and mixtures thereof. As stated therein, the addition of said cleaning agent enhances the cleaning power of sodium or potassium hydroxide to remove rolling oil, tramp mill oil and steel fines. Iron sequestering agents may also be added to this bath for preventing or removing steel tarnishes. The reference, as a whole though, neither teaches nor suggests any use of HEDP for inhibiting the formation of magnesium oxide-containing film on aluminum-magnesium alloy sheet product.